Robert Plant's towering vocal chops were a monument of ‘70s rock. Later, the flamboyant singer expanded his sonic palette, lending his otherworldly pipes to ‘80s synth-pop like “Walking Towards Paradise”; “Pledge Pin” is sax-kissed pop-rock indulgence. Then there's 2005's proggy “Tin Pan Valley,” which proves that the mystique of his murmur has been burnished with time.